The interface formation of the organic semiconductor CuPcF4 with both polycrystalline and single crystalline silver substrates has been investigated using photoemission spectroscopy. We observe an electron transfer from the both the Ag substrates to the CuPcF4 molecules at the interfaces. Furthermore we observe a change of the spectral shape of the highest occupied molecular orbital (HOMO) of CuPcF4 as a function of the layer thickness. Especially for submono- and monolayers of CuPcF4 on single crystalline Ag we resolve a double peak structure of the HOMO, which is discussed in detail. We compare the character of the charge transfer from the silver to the CuPcF4-layer with the electron transfer from K to CuPcF4 within a thick K-intercalated CuPcF4-layer. (C) 2003 Elsevier B.V. All rights reserved.